Rhizogeton sp.

Calder, Dale R., 2010, Some anthoathecate hydroids and limnopolyps (Cnidaria, Hydrozoa) from the Hawaiian archipelago 2590, Zootaxa 2590 (1), pp. 1-91 : 13-14

Rhizogeton sp.

Fig. 4

(?) Rhizogeton sp. — Cooke, 1977: 83, fig. 11.

Material examined. Oahu : Kaneohe Bay, south of Kekepa Island, 2 m, 10.iv.1979, on Porites compressa in alpheid crevice, 13 colony fragments, without gonophores, coll. W.J. Cooke, BPBM (without catalog number) .– Oahu : Kaneohe Bay, south of Kekepa Island, 2 m, 10.iv.1979, on Porites compressa in alpheid crevice, several hydranths from sample above, without gonophores, coll. W.J. Cooke, ROMIZ B3833 .

Description. Hydroid colonies stolonal, arising from a creeping hydrorhiza usually obscured by algal mat on coral substrate. Perisarc of hydrorhiza thin, translucent, smooth to wrinkled but not annulated, extending over base of hydranth as an almost invisible, filmy, close-adhering sheath (apparent after dissolving a hydranth in sodium hypochlorite). Hydranths essentially sessile, clavate to elongate and almost cylindrical, when extended up to 4 mm long, 0.25 mm wide; tentacles filiform, up to about 25 in number, scattered over distal half to three-quarters of hydranth, distal ones longer, thicker, and closer together than those basally; hypostome conical to proboscis-like. Colour of live hydranths brownish-red, with distinct white spots located at bases of tentacles.

Gonophores not seen.

Remarks. Hydroids assigned to Rhizogeton L. Agassiz, 1862 (as Rhizogeton sp. and R. nudum Broch, 1909 ) have been reported from a number of locations in warm waters of the Pacific and Indian oceans, including Mozambique ( Millard & Bouillon 1974; Millard 1975), La Réunion ( Gravier-Bonnet & Mioche 1996), south India ( Mammen 1963), Christmas Island ( Ritchie 1910b), Enewetak Atoll ( Cooke 1975), and Hawaii ( Cooke 1977). It is improbable that the cold-water R. nudum is present in the tropical and subtropical Indo- Pacific, and records of the species from the region are believed to be based on misidentified material. Hirohito (1988) reported that specimens from Sagami Bay, Japan, identified as R. ezoense Yamada, 1964 (type locality: Hokkaido, Japan), resembled those of Millard & Bouillon (1974) from Mozambique.

A hydroid species identified as Rhizogeton sp. is frequent in Hawaii inside alpheid crevices on the scleractinian corals Porites lobata and P. compressa ( Cooke 1977; personal observations). Gonophores have yet to be discovered in material from the region. The species is possibly identical with hydroids identified as Rhizogeton sp. from shrimp crevices on reef flats in the Indian Ocean at La Réunion by Gravier-Bonnet & Mioche (1996). Their hydrozoan is now considered an undescribed species, fertile colonies have been found, and it has been collected on other islands in the Indian Ocean (Nicole Gravier-Bonnet, pers. comm., 3 July 2009). The coral substrates of the hydroid in Hawaii are abundant and widespread ( Maragos 1977).

Reported distribution. Hawaii. No location given, on Porites lobata , in alpheid crevices ( Cooke 1977). Material examined during this study was collected around Oahu : from Kekepa Island (10.iv.1979, coll. W.J. Cooke); other specimens were observed on similar substrates at Haleiwa Beach (12.vii.2009), and off Kailua Beach (26.vii.2009) (personal observations) .